BIZOMAT MAKEOS · Quality
Dovetail Inspection over Rods (Reading Limits, Accept or Reject)
What reading over (or between) rods should this dovetail give at its width limits, and does my measured reading pass?
Inspection of a male or female dovetail with two precision rods: the reading over or between the rods for the nominal width and at both width limits (as the dovetail measurement tool), the width worked back from your measured reading, whether the rods contact the flanks within the dovetail depth, and accept or reject against the width tolerance.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Dovetail type | choice | Male tongue (measure over rods) or female slot (measure between rods). |
| Dovetail angle (flank to base) | ° | Angle between the flank and the base. |
| Nominal width at the base corners | mm | Nominal width at the corners where the flanks meet the base. |
| Upper width deviation | mm | Upper deviation of the width. |
| Lower width deviation | mm | Lower deviation of the width. |
| Rod diameter | mm | Diameter of the two rods. |
| Dovetail depth | mm | Depth of the dovetail (the rods must touch the flanks within it). |
| Measured reading | mm | Micrometer reading over (or between) the rods. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Dovetail type
- What it is
- Whether the part is a male dovetail (a tongue, measured over two rods with a micrometer) or a female dovetail (a slot, measured between two rods).
- Units and format
- Choose one of 2 options from the list: Male (tongue) – over rods; Female (slot) – between rods.
- Example
- The form opens with “Male (tongue) – over rods” selected as a worked example. Choose the option that matches your case.
- Get it right
- The choice reverses the sign of the rod correction, so check it before reading the result.
What this means: Dovetail angle (flank to base)
- What it is
- The angle between a sloping flank of the dovetail and its base surface, as dimensioned on the drawing.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 1° to 89.999°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the flank-to-base angle, not the angle of the flank from vertical.
What this means: Nominal width at the base corners
- What it is
- The nominal width of the dovetail measured between the sharp theoretical corners where the flanks meet the base.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- If the drawing gives the width at the top, convert it to the width at the base corners first.
What this means: Upper width deviation
- What it is
- How far above nominal the width may be, from the drawing tolerance. The nominal width plus this deviation gives the upper limit.
- Units and format
- Enter a number in mm: millimetres. The form accepts -10 mm to 10 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.05 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Type a negative value if the whole tolerance band is below nominal.
What this means: Lower width deviation
- What it is
- How far below nominal the width may be, from the drawing tolerance. The nominal width plus this deviation gives the lower limit.
- Units and format
- Enter a number in mm: millimetres. The form accepts -10 mm to 10 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with -0.05 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- A deviation below nominal is negative; keep the minus sign.
What this means: Rod diameter
- What it is
- The diameter of the two precision rods or gauge pins placed in the dovetail corners. Both rods must have the same diameter.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The reading changes with rod size times the rod factor, so a wrong rod size shifts the width directly.
- Get it right
- Use the measured or certified rod size, not a nominal one.
What this means: Dovetail depth
- What it is
- The depth of the dovetail from its top surface down to the base. The rods must touch the flanks below this height for a valid reading.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 12 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- If the rods would touch above it, choose smaller rods.
What this means: Measured reading
- What it is
- The micrometer reading taken across the outside of both rods for a male dovetail, or between the rods for a female one.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 77.33 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The width worked back from it changes one for one with the reading.
- Get it right
- Make sure both rods sit fully in the corners and the micrometer is square to them.
Method and formulas
- Rod factor k = 1 + cot(angle ÷ 2); reading = width + rod × k (male) or width − rod × k (female) (as the dovetail measurement tool).
- Width from a reading = reading − rod × k (male) or reading + rod × k (female).
- Rod contact height = (rod ÷ 2) cot(angle ÷ 2) sin(angle), which must be below the depth.
- Example: the reading should lie between 77.271 and 77.371 mm; 77.33 mm means a width of 50.009 mm.
Limitations
- Sharp corners at the base; corner radii or reliefs do not affect the rod reading but do affect the width definition.
- Rod diameter errors enter the result multiplied by the rod factor.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Inspect a dovetail slide or gib
- Put reading limits on a drawing
- Check whether the chosen rod size is suitable
Who it is for
- Inspectors
- Toolmakers
- Machinists
What you get
- Reading at nominal and at the width limits
- Width from the measured reading
- Rod contact and the decision
Interpreting the result
- Choose rods that touch the flanks near mid-depth: too small and burrs at the base interfere, too large and they ride over the top.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 682. Topic reference; no source tables or recommended settings are reproduced.
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